The invention is not limited to any certain applications, but as an example, we discuss the technology of LED replacement light bulbs. Due to poor efficiency of incandescent lighting there is a tendency to increase the use of LEDs instead of incandescent light bulbs. Incandescent light bulbs can be easily replaced by using LED replacement light bulbs. They are electrically and mechanically compatible with incandescent light bulbs but use less electrical energy for producing a certain luminous flux.
FIG. 1 illustrates main parts of an exemplary prior art LED replacement light bulb. It has three LEDs 21, 22, 23, which are mounted on a MCPCB (Metal Core Printed Circuit Board) 30. The MCPCB is electrically connected to a power supply 50 with wires 31, 32. The power supply is further connected to light fitting 60, which is an electrical and mechanical interface of the device. The LEDs generate heat which must be dissipated efficiently in order to keep the temperatures of the LEDs at a moderate level. FIG. 1 illustrates a heat sink 41, which is made of aluminium, for example. The MCPCB is attached to the heat sink with temperature interface material. The heat dissipated from the LEDs is first conducted to the MCPCB and further to the heat sink. The device also has a translucent cover 70.
There are certain problems related to the prior art solutions for dissipating heat from electrical components such as LEDs. The dissipation of heat is not efficient when the heat is conducted through several thermal interfaces between different parts, such as LED—MCPCB—heat sink, and interface materials in between, such as thermal interface materials like silicon grease between the MCPCB and heat sink. Heat conductivity of a MCPCB is also relatively low because the heat is conducted through insulating dielectric layer i.e. MCPCB dielectric laminate. Due to the inefficient heat dissipation the opto-electronic components, such as LEDs, cannot be used with a maximum or even with a high power. Therefore, in order to achieve a certain power level, it is required to provide larger amount of opto-electronic components, which also adds to the production cost of a device. The inefficient thermal connection limits also the integration density of LEDs, i.e. due to inefficient thermal connection LEDs close to each other start to heat up each other.
The more inefficient i.e. higher the thermal resistance of connection between the component and heat sink is, the smaller thermal resistance of the heat sink can be, which means relatively large heat sinks or active cooling. Therefore it is sometimes not possible to design the devices into a desired size. Prior art heat sinks are generally made of metal, such as aluminium. In order to raise the emissivity it is beneficial to provide anodizing or other kind of coating for the metal heat sink, which causes costs. The cost of the heat sink and MCPCB is relatively high compared to other components of a LED replacement bulb, for example.
It is a known solution to use plastic heat sinks. However, the interconnection of electrical structures to plastic heat sinks uses MCPCB or other PCB solutions attached with some thermal interface material to plastic heat sinks, where some solution needs to be provided, in order to provide planar surfaces in between the MCPCB and the heat sink. Planar surface in between is needed to provide efficient thermal connection.
A further disadvantage of the prior art assemblies relates to complicated production processes. It is typically necessary to connect a MCPCB to a power supply with wires and to use manual assembly with possible thermal interface materials for attaching the MCPCB with a heat sink. When the heat sink assembly includes a heat sink made of aluminium as well as a PCB the weight of the heat sink assembly tends to become high.
A typical LED replacement bulb solution assembled to a standardized lamp socket has a problem of providing poor and/or disturbed air flow for the heat sink of the replacement bulb. This is due to the optical surface also requiring large surface area, which is thus reduced from the heat dissipating area. The optical surface area is needed to meet the requirements set for the replacement bulbs. Requirements include e.g. omni directionality. Another air flow disturbing and heat dissipating surface minimizing factor is the volume that the power supply requires in the standardized socket LED replacement solutions. This volume reservation reduces the volume available for arranging the heat dissipating surface. Also, the standardized base is a closed structure, not allowing air to flow. Another problem with the standard base/socket is poor conduction of heat through the base/socket to the luminaire structure.